Digging Into Mac OS X’s Single-Application Mode

Way back, when we were recovering from our hangovers from the millennium parties, Apple introduced, for the first time, Mac OS X and the Aqua user interface. This was still a preview, so it wasn’t quite as polished and finished, of course. It also contained a feature that never made it into the final releases: single-window mode. Or did it…?

Single-application mode

We’re speaking MacWorld San Francisco, January 2000. We’re all recovering from our debilitating millennium parties, or, depending on your score on the ‘n’ in OCEAN, trying to dig your way out of your emergency shelter. Apple was wrapping up the development of its next generation operating system, which was based on NeXTSTEP, and at MacWorld that year, Steve Jobs unveiled the new Aqua user interface for the first time.

He showed off all the things that we take for granted now in Mac OS X; sheets, the genie effect, shadows, transparency, the dock, magnification, and so on, and so forth. One of the features Steve Jobs made a pretty big deal of was the purple button. It would be reviled and hated afterwards, but the crowd cheared during the demonstration.

The purple button was a widget located on the far right side of every window’s title bar. It looked just like the red, yellow and green buttons, except that it was purple. The function of this widget was to switch between regular mode and what Steve Jobs called ‘single-window mode’.

When single-window mode was active (the widget was a toggle), only one window would be visible at any given moment. For instance, if you had three window open – a Finder window, a photo in Preview, and a document in TextEdit – you would only ever see one. If Preview was active, and you switched to TextEdit, Preview would minimise to the dock, and TextEdit would appear, both using the genie effect. Jobs claimed this feature was for new users, but that advanced users would love it too.

It was not particularly loved by testers of the early preview releases of Mac OS X. Apart from whether or not it was useful, people said the implementation was wrong. In his February 2000 review of Mac OS X Developer Preview 3, John Siracusa wrote that such a function, whether it is applicable to a small group of people or every user, should not be located on every window throughout the entire operating system. Siracusa likened it to having a system-wide volume control located in every window.

Speaking of DP3, if you have a copy lying around, install it on an old G3 or something and hit ctrl+alt+delete in the ‘About Mac OS’ window. Hilarity ensues.

In any case, as soon Mac OS X hit public beta, the purple widget was gone, and so was single-window mode. Or was it? Lewis Butler did some digging around, and found out that single-window mode still exists in Mac OS X today – albeit in a slightly different form. You can enable it using the following terminal commands:

defaults write com.apple.dock single-app -bool true<br />
killall Dock

To disable:

defaults delete com.apple.dock single-app<br />
killall Dock

You’re now running in what’s left of single-window mode. As you can deduce from the terminal command, this is strictly speaking not single-window mode; it’s actually single-application mode. This means that instead of ever only one window being visible, you’ll have one application visible at all times – including all of its child windows. This means that the genie effect will no longer be used, so application switching is all a bit jarring, especially if you have multiple windows open per application.

The “feature” is not very pervasive; it’s completely tied to the dock. Only if you switch through the dock will you see the detailed behaviour. If you switch via any other means, such as Exposé, command+tab, or even clicking another visible application’s window, Mac OS X’s regular multi-application approach takes over.

I actually played with this feature for a while on my non-Apple labelled computer (it runs Plex as a media centre in the living room, connected to my TV), and I must say that it is actually kind of useful. I think it really depends on what your regular means of application switching and starting is; if you use the dock for that, you’ll find this feature annoying. If you are like me, and use Exposé for switching and command+space for application loading, and never really touch the dock, then this feature turns the dock into a handy anti-clutter device.

I’m guessing it can also be handy for those of you who choose to exercise your rights by running Mac OS X on a netbook. On such a small screen, this single-application mode could prove to be useful.

Window layering

As a side-note, the single-window mode versus single-application mode is an interesting extension of the debate that reared its head when Mac OS X first arrived in the hands of eager testers. In Mac OS 9 and earlier, windows were layered in groups based on application; in Mac OS X, this changed to layering on a per-window basis. A lot of Mac users were not particularly thrilled about this change.

When windows are layered per application, all child windows of an application will be brought forward when you switch to that application Рnot just the window you clicked on. Mac OS X is currently a bit schizophrenic about the whole thing; clicking on a dock icon will bring all its child windows to front, whereas using Expos̩ or clicking on a window will not.

When windows are layered per window, it means that windows belonging to different applications will interleave with one another. This can most certainly be more confusing, because it presents you with no option to narrow your search for a particular window. In a per-window layered environment, the list of windows is a linear one, whereas in a per-application environment, any click inside any window will reduce the number of window choices dramatically. If you regularly use a lot of windows belonging to a lot of different applications, there might be something to say for per-application based window layering.

This little bit of computer archaeology is quite fascinating. Who knows what other features lie hidden beneath that candy-coated Aqua surface?


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